1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a magneto-optical recording method, in which information is recorded by forming bits of reversed magnetic domains using a laser beam and an external magnetic field, and the information is read out by utilizing a magneto-optical effect by irradiating a polarized laser beam and, more particularly, to a magneto-optical recording medium and a recording method, which can improve the data transfer rate by verifying recorded information simultaneously with information recording.
2. Related Background Art
As a rewritable high-density recording system, a magneto-optical recording medium on which information is recorded by writing magnetic domains in a magnetic thin film using thermal energy of a semiconductor laser, and from which the information is read out using a magneto-optical effect, has received a lot of attention.
In recent years, a demand has arisen for increasing the recording speed of the magneto-optical recording medium.
In the magneto-optical recording method, the most basic technique of rewriting information includes the following three processes performed on a rotating disk-shaped recording medium.
(1) A DC external magnetic field is applied in an erasing direction, and a continuous laser beam is irradiated onto a position to be subjected to recording, thereby erasing old information. PA1 (2) A DC external magnetic field is applied in a recording direction, and a laser beam modulated in accordance with recording information is irradiated onto the position to be subjected to recording, thereby recording information. PA1 (3) Finally, a continuous weak laser beam is irradiated onto the position to be subjected to recording to read out recorded information, and it is confirmed if the information is normally recorded (verifying operation). PA1 a substrate; PA1 a first magnetic layer which is formed on the substrate and has an in-plane magnetization film at room temperature; PA1 a second magnetic layer which is formed on the first magnetic layer and has a Curie temperature lower than a Curie temperature of the first magnetic layer; and PA1 a third magnetic layer which is formed between the first and second magnetic layers, and has a Curie temperature lower than the Curie temperatures of the first and second magnetic layers, PA1 wherein the first magnetic layer changes to a perpendicular magnetization film at a temperature between room temperature and the Curie temperature of the third magnetic layer. PA1 a substrate; PA1 a first magnetic layer which is formed on the substrate and has an in-plane magnetization film at room temperature; PA1 a second magnetic layer which is formed on the first magnetic layer and has a Curie temperature lower than a Curie temperature of the first magnetic layer; and PA1 a third magnetic layer which is formed between the first and second magnetic layers, and has a Curie temperature lower than the Curie temperatures of the first and second magnetic layers, and PA1 in which the first magnetic layer changes to a perpendicular magnetization film at a temperature between room temperature and the Curie temperature of the third magnetic layer, PA1 the method comprising the steps of: PA1 irradiating a recording laser spot, PA1 wherein the first magnetic layer has an inplane magnetization in a low-temperature region in the spot, and the first magnetic layer has a perpendicular magnetization and a temperature of the third magnetic layer exceeds a Curie temperature to cut off exchange coupling between the first and second magnetic layers in a high-temperature region of the spot; PA1 applying an external magnetic field modulated according to recording information to record information in the second magnetic layer, and orienting a direction of magnetization, in the high-temperature region, of the first magnetic layer in a direction of the external magnetic field; and PA1 verifying the information by detecting reflected light, from the medium, of the recording laser spot.
In the magneto-optical recording medium, since information cannot often be normally recorded due to defects, deterioration, or corrosion of the recording medium, dust, failure of a magneto-optical recording apparatus, and the like, the verifying operation of recorded information in process (3) is indispensable.
As can be seen from the above description, rewriting information requires the time it takes for three revolutions of the disk. If this operation can be achieved by two revolutions of the disk or one revolution of the disk, a high-speed processing upon rewriting information can be realized.
In order to achieve this, various methods have been proposed. For example, in order to simultaneously perform processes (1) and (2) (i.e., perform them in one revolution of the disk), a method of irradiating a continuous laser beam under an external magnetic field modulated according to recording information (magnetic field modulation overwrite), a method of irradiating a laser beam which is modulated to high and low levels in accordance with recording information under a DC external magnetic field using a special medium (optical modulation overwrite), and the like have been proposed.
In order to perform process (3) simultaneously with processes (1) and (2), a method in which two laser beams are used in a single optical system, so that recording is performed by the first beam, and recorded information is verified by the second beam, has been proposed. When all the above-mentioned methods are used, processes (1), (2), and (3) can be completed within one revolution of the disk.
When the verifying operation of recorded information in process (3) is achieved by using two laser beams in a single optical system, the optical system is complicated. In addition, since the optical system requires high mechanical precision, it becomes bulky and complex, and its cost increases.
Thus, a method of performing the verifying operation in process (3) simultaneously with the magnetic field modulation overwrite using a single laser beam, thereby completing processes (1), (2), and (3) within one revolution of the disk, has been disclosed. For example, Japanese Laid-Open Patent Application No. 3-207040 or 5-20720 has proposed a method in which a magneto-optical recording medium comprising a two-layered magnetic film is used, and the verifying operation of recorded information is performed simultaneously with recording using light reflected by a magnetic layer having a high Curie temperature. On the other hand, Japanese Laid-Open Patent Application No. 5-182269 has proposed a method of similarly performing the verifying operation simultaneously with recording by arranging an intermediate layer with a low Curie temperature between a reproducing layer and a recording layer.
In recent years, a demand has arisen for a large-capacity recording medium by increasing the recording density in addition to an increase in recording speed of the magneto-optical recording medium. In order to meet such a demand, a technique for improving the recording density by magnetic super-resolution by modifying the structure of the recording medium and the reading method has been developed. For example, Japanese Laid-Open Patent Application No. 3-93058 has proposed to increase the line recording density and the track density in such a manner that a medium comprising a reproducing layer and a recording layer is used, and after the direction of magnetization of the reproducing layer aligns in one direction using an initialization magnetic field before reproduction of a signal, a signal held on the recording layer is transferred to the reproducing layer while applying a reproduction magnetic field so as to reduce any intersymbol interference upon reproduction, thus allowing reproduction of a signal having a period equal to or smaller than the diffraction limit of light.
However, with the above-mentioned verifying method, it is difficult to perform a reliable verifying operation, and to reproduce information recorded at a high density. The reasons for these problems will be explained below.
FIGS. 1A to 1C respectively show the light spot, magnetization state, and temperature distribution obtained when the verifying operation described in Japanese Laid-Open Patent Application No. 3-207040 or 5-20720 is performed. FIGS. 2A to 2C respectively show the light spot, magnetization state, and temperature distribution obtained when the verifying method described in Japanese Laid-Open Patent Application No. 5-182269 is executed. FIGS. 1A and 2A show a generally circular light spot incident on a recording track, with an old information detection region being shaded and a new information detection region being unshaded.
As shown in FIGS. 1B, 1C or 2B, 2C, the temperature distribution formed upon irradiation of a laser beam onto a magneto-optical recording medium normally extends in the traveling direction of a medium. FIGS. 1B or 2B show a laser beam incident on a reproducing layer side, while a recording magnetic field is incident on the recording layer side of the medium. More specifically, the temperature of a medium portion in a region, opposite to the medium traveling direction, in the light spot does not rise very much. It is difficult to raise the temperature of this low-temperature region to a temperature near the Curie temperature of the recording layer due to the upper limits of the thermal diffusion rate in the medium and the laser power. Therefore, the light spot always includes a region where the temperature of the recording layer portion does not reach its Curie temperature, as shown in FIGS. 1B or 2B, and magnetization of old information remains in this recording layer portion.
In order to realize a state wherein the direction of magnetization of the reproducing layer aligns in the direction of an external magnetic field to form a magnetic wall at an interface with the recording layer while the reproducing layer and the recording layer have interface magnetic wall energies high enough to apply an exchange coupling force capable of magnetic transfer therebetween, a magnetic field as large as 3 kOe or more is normally required as the external magnetic field. For this reason, the magneto-optical recording apparatus must have a large-size magnet, and it is impossible for a floating type magnetic head, normally used in the magnetic field modulation overwrite, to apply such a large magnetic field.
For this reason, in the magneto-optical recording medium and the verifying method described in Japanese Laid-Open Patent Application No. 3-207040 or 5-20720, an exchange coupling force acts between the reproducing layer and the recording layer in the low-temperature region in the light spot, thus forming a region where old information is transferred to the reproducing layer, as shown in FIG. 1B. Therefore, as shown in FIG. 1A, when the track width is decreased, the old information is detected simultaneously with new information as well as information or neighboring tracks in the verifying operation. For this reason, it is difficult to obtain a reliable verification signal. Furthermore, with this method, it is almost impossible to simultaneously realize the super-resolution effect.
In the magneto-optical recording medium and the verifying method described in Japanese Laid-Open Patent Application No. 5-182269, as well, since old information is detected in the low-temperature region before the temperature of the intermediate layer reaches its Curie temperature, as shown in FIG. 2A showing old and new information detection regions, and in FIG. 2B showing the magnetization state, it is difficult to obtain a reliable verification signal. With this method, although the super-resolution effect for increasing the line recording density can be realized, it is impossible to increase the recording density in the track direction, and the magneto-optical recording apparatus becomes complicated.
Furthermore, in the method of Japanese Laid-Open Patent Application No. 3-93058 proposed as a method of realizing the super-resolution, an initial magnetic field as large as about 3 kOe before reproduction, and a reproduction magnetic field are required, resulting in a bulky, high-cost magneto-optical recording apparatus.